I am trying to set it up so that I can access my home network via a VPN connection, but I am getting the following error:

Error Message

"Error 809: The network connection between your computer and the VPN server could not be established because the remote server is not responding. This could be because one of the network devices (e.g. firewalls, NAT, routers, etc.) between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator or your service provider to determine which device may be causing the problem".

Router

Verizon FiOS with a dynamic IP address (DynDNS).
Model: MI424WR (Rev 2)
The router does not handle DHCP or DNS (DHCP is handled by NAS and DNS is handled by domain server).
I have forwarded ports 1723 and 1701 to the NAS.
I cannot find a setting for 'VPN passthrough' in the router's configuration.

I have tried setting up the NAS to be linked as a DMZ, but it didn't seem to change anything.